Paper webs or sheets, sometimes called tissue or paper tissue webs or sheets, find extensive use in modern society. Such items as paper towels, napkins, and facial tissues are staple items of commerce. It has long been recognized that three important physical attributes of these products are their softness; their absorbency, particularly their absorbency for aqueous systems; and their strength, particularly their strength when wet. Research and development efforts have been directed to the improvement of each of these attributes without deleteriously affecting the others as well as to the improvement of two or three attributes simultaneously.
Softness is the tactile sensation perceived by the consumer as he/she holds a particular product, rubs it across his/her skin, or crumples it within his/her hand. This tactile sensation is a combination of several physical properties. One of the more important physical properties related to softness is generally considered by those skilled in the art to be the stiffness of the paper web from which the product is made. Stiffness, in turn, is usually considered to be directly dependent on the dry tensile strength of the web.
Strength is the ability of the product, and its constituent webs, to maintain physical integrity and to resist tearing, bursting, and shredding under use conditions, particularly when wet.
Absorbency is the measure of the ability of a product, and its constituent webs, to absorb quantities of liquid, particularly aqueous solutions or dispersions. Overall absorbency as perceived by the human consumer is generally considered to be a combination of the total quantity of liquid a given mass of tissue paper will absorb at saturation as well as the rate at which the mass absorbs the liquid.
The use of wet strength resins to enhance the strength of a paper web is widely known. For example, Westfelt described a number of such materials and discussed their chemistry in Cellulose Chemistry and Technology, Volume 13, at pages 813-825 (1979).
Freimark et al. in U.S. Pat. No. 3,755,220 issued Aug. 28, 1973 mention that certain chemical additives known as debonding agents interfere with the natural fiber-to-fiber bonding that occurs during sheet formation in papermaking processes. This reduction in bonding leads to a softer, or less harsh, sheet of paper. Freimark et al. go on to teach the use of wet strength resins to enhance the wet strength of the sheet in conjunction with the use of debonding agents to off-set undesirable effects of the wet strength resin. These debonding agents do reduce dry tensile strength, but there is also generally a reduction in wet tensile strength.
Shaw, in U.S. Pat. No. 3,821,068, issued Jun. 28, 1974, also teaches that chemical debonders can be used to reduce the stiffness, and thus enhance the softness, of a tissue paper web.
Chemical debonding agents have been disclosed in various references such as U.S. Pat. No. 3,554,862, issued to Hervey et al. on Jan. 12, 1971. These materials include quaternary ammonium salts such as trimethylcocoammonium chloride, trimethyloleylammonium chloride, dimethyl-di(hydrogenated-tallow)ammonium chloride and trimethylstearylammonium chloride.
Emanuelsson et al., in U.S. Pat. No. 4,144,122, issued Mar. 13, 1979, teach the use of complex quaternary ammonium compounds such as bis(alkoxy-(2-hydroxy)-propylene) quaternary ammonium chlorides to soften webs. These authors also attempt to overcome any decrease in absorbency caused by the debonders through the use of nonionic surfactants such as ethylene oxide and propylene oxide adducts of fatty alcohols.
Armak Company, of Chicago, Ill., in their bulletin 76-17 (1977) disclose that the use of dimethyldi(hydrogenated-tallow)ammonium chloride in combination with fatty acid esters of polyoxyethylene glycols may impart both softness and absorbency to tissue paper webs.
One exemplary result of research directed toward improved paper webs is described in U.S. Pat. No. 3,301,746, issued to Sanford and Sisson on Jan. 31, 1967. Despite the high quality of paper webs made by the process described in this patent, and despite the commercial success of products formed from these webs, research efforts directed to finding improved products have continued.
For example, Becker et al. in U.S. Pat. No. 4,158,594, issued Jan. 19, 1979, describe a method they contend will form a strong, soft, fibrous sheet. More specifically, they teach that the strength of a tissue paper web (which may have been softened by the addition of chemical debonding agents) can be enhanced by adhering, during processing, one surface of the web to a creping surface in a fine patterned arrangement by a bonding material (such as an acrylic latex rubber emulsion, a water soluble resin, or an elastomeric bonding material) which has been adhered to one surface of the web and to the creping surface in the fine patterned arrangement, and creping the web from the creping surface to form a sheet material.
It is an object of this invention to provide a process for making soft, absorbent tissue paper webs with high permanent wet strength.
It is a further object of this invention to provide soft, absorbent paper towel products with high permanent wet strength.
These and other objects are obtained using the present invention, as will become readily apparent from a reading of the following disclosure.